MIC2580A Hot-Swap PCI Power Controller - Micrel - #19

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February 2005 19 MIC2580A MIC2580A Micrel, Inc. Application Information Whenever voltage is applied to a highly capacitive load, high inrush currents may result in voltage droop that may bring the supply voltage out of regulation for the duration of the transient. The MIC2580A solves this problem by specifically controlling the current and voltage supply ramps so that the system supply voltages are not disturbed. Very large capacitive loads are easily supported with this device. Figure 1 shows the timing during turn-on. When /ON is forced low, all supplies are turned on at a slew rate determined by the external capacitor, CSLEW. Figure 2 shows the foldback characteristics for the supply voltages. This foldback affect bounds the magnitude of the current step when the supplies are turned on or shorted. This specifies the compact PCI specification of 1.5A/ms, thereby ensuring reliable operation. In discrete FET implementations, this magnitude can exceed several amps and may cause the main supply to go out of regulation during this transient event. This, in turn, could cause the system to behave unpredictably. In addition, should a fault occur, the MIC2580A will prevent system malfunctions by limiting the current to within specifications. MOSFET Selection The external MOSFET should be selected to provide low enough dc loss to satisfy the application’s voltage regulation requirements. Note that the voltage across the sense resistor must also be added to the dc voltage drop across the MOSFET to compute total loss. In addition to meeting the voltage regulation specifications, thermal specifications must also be considered. During normal operation very little power should be dissipated in the MOSFET. DC power dissipation of the MOSFET is easily computed as I2RDS where I is the drain current and RDS is the specified on-resistance of the MOSFET at the expected operating drain current. However, during excessive drain current or short-circuit faults, the power dissipation in the external MOSFET will increase dramatically. To help compute the effective power dissipated during such transients, MOSFET manufacturers provide transient thermal impedance curves for each MOSFET. These curves provide the effective thermal impedance of the MOSFET under pulsed or repetitive conditions; for example, as will be the case when enabling into a short circuit fault. From these curves the effective rise in junction temperature of the MOSFET for a given condition can be computed. The equation is given as: peak TJ = PDM × ZèJA + TA where PDM is the power dissipated in the MOSFET usually computed as VIN x IDRAIN and ZèJA is the thermal response factor provided from the curves. Since the MIC2580A reduces the current to 30% of full scale even under severe faults such as short-circuits the MOSFET power dissipation is held to safe levels. This feature allows MOSFETs with smaller packages to be used for a given application thereby reducing cost and PCB real-estate requirements. 12VOUT 5VGATE 3VGATE VPCHG 12VIN /FAULT GND CSTART /LPCIRST 5VIN 3VIN /PCIRST /EPWDGD /ON CSLEW MIC2580A 5VSENSE 3VSENSE 5VOUT 3VOUT CRST IRF7413 IRF7413 3.3V 5V 12V /PWDGD M12VIN M12VOUT +12V +5V +3.3V GND –12V Power Supply /CIRST BUS EN –12V /CIRST Data Bus /CIRST –12V /100mA 12V/500mA 5V/5A 3.3V/7.6A 10mÙ 10mÙ PCI Hot-Plug Controller Bus Switch Data Bus GND Adapter Platform /POR D0 D0 +5V D1 D1 D2 D2 Dn Dn Figure 6. Hot-Plug PCI Application